1. Field of the Invention
The present invention relates to an apparatus and a method for stripping silicon nitride, and more particularly, to an apparatus and a method for stripping silicon nitride that facilitate automatic, real-time, and exact measurement of etch rate and an ending time of the etching process when silicon nitride is stripped with phosphoric acid solution.
2. Discussion of the Related Art
IC has devices including transistors and diodes formed on a semiconductor substrate, and metal lines connecting the devices with each other. An insulating layer is formed for various purposes, e.g., to insulate between the metal lines and to improve the characteristics of the devices.
The insulating layer includes silicon oxide (SiO2), doped silicon oxide, and silicon nitride such as SiN and Si3N4.
The silicon nitride is used as an insulating layer for a capacitor device, a passivation layer for preventing diffusion of moisture and sodium, a mask for selective etching since the silicon nitride is not oxidized and thus prevents a silicon oxide layer thereunder from being oxidized, and an etch stop layer for a silicon oxide layer thereon or thereunder.
After deposited, the silicon nitride is patterned or stripped for a certain purpose. The patterning or stripping process is usually performed using wet etch method with ortho-phosphoric acid (H3PO4).
The chemical reaction during the wet etch process is represented by the chemical equation (1) as follows:3Si3N4+27H2O+4H3PO4→4(NH4)3PO4+9H2SiO3.  (1)
As shown in the chemical equation (1), phosphoric acid and water are consumed during the stripping process of a silicon nitride. The phosphoric acid in the phosphoric acid solution is dehydrated, the chemical equations of which are as follow:2H3PO4→H4P2O7+H2O  (2)H4P2O7→2HPO3+H2O  (3)2HPO3→P2O5+H2O  (4)
The phosphoric acid (H3PO4) produces phosphorus pentoxide (P2O5) through the dehydration reaction of chemical equation (2) through (4), during which pyro-phosphoric acid (H4P2O7) and meta-phosphoric acid (HPO3) are formed respectively as an intermediate.
Through the dehydration reaction, the concentration of water and phosphoric acid decreases and the etch rate of silicon nitride reduces. Meanwhile, it is usual to keep adding water since more water is consumed than phosphoric acid while silicon nitride is stripped.
Continuous supply of water, however, may cause re-hydration reaction, and thus, variation of etch rate of silicon nitride. The variation of etch rate makes it hard to know the thickness of the silicon nitride etched and to find out the time when the etching process should be stopped. Furthermore, if too much water is added, there may occur problems, e.g., the etch rate is highly reduced. Thus, a method has been required that facilitates automatic, real-time, and exact measurement of etch rate and an ending time of the etching process when silicon nitride is stripped.
U.S. Pat. No. 4,092,211 by Morris discloses a method for controlling etch rate of silicon oxide by adding silicate materials to a phosphoric acid solution when silicon oxide used for masking silicon nitride is selectively removed with the phosphoric acid solution. U.S. Pat. No. 5,938,885 by Huang et al. discloses a method for continuously monitoring etch rate of silicon nitride with hydrometer and supplying water or heat to control the etch rate when the silicon nitride is etched with phosphoric acid solution.
The method disclosed by Huang et al., however, is not a method for quantitatively measuring the etch rate of the silicon nitride but merely a method for supplying water or heat which is reduced during the etching process of the silicon nitride.